Alkyl branched fatty acids occur as a by-product of the catalytic or thermal dimerisation of unsaturated straight chain fatty acids. Such alkyl branched fatty acids are known as “isostearic acid”. Isostearic acid is liquid at ambient temperature, exhibits better stability to oxidation than oleic acid, and consequently is a very useful product which is sold Into a wide range of application areas such as lubricant esters, and cosmetic applications. Isostearic acid is also used to make isostearyl alcohol. Isostearic acid is produced commercially by the so-called Emersol process.
In the Emersol process the fatty acid mixture is dissolved in organic solvent and then cooled down using horizontal scraped crystallisers. The linear fatty acid crystals which are formed during cooling are removed from the unsaturated linear fatty acids, unsaturated branched fatty acids and saturated branched fatty acids with a rotary drum filter. The solvent is then removed from both the fractions in a distillation step. The Emersol process is energy intensive, has high operational costs, and requires the use of undesirable organic solvent.
U.S. Pat. No. 4,973,431 A describes a wet separation process for the production of isostearic acid. In the wet separation process, the fatty acid mixture is also cooled down using horizontal scraped crystallisers. But then, a wetting agent solution, which is a mixture of water and surfactant, is added to the fatty acids. The linear fatty acid crystals are suspended in the aqueous phase and then separated from the mixture with a cold centrifuge. The linear fatty acids are then heated, breaking up the suspension and separated from the surfactant in a hot centrifuge. The use of both types of centrifuges results in high maintenance costs. The use of surfactant further increases the cost of the wet separation process.
A third technology, known as dry fractionation has been used to separate saturated fatty acids from unsaturated fatty acids, but it has not been applied to the separation of branched fatty acids. Dry fractionation involves the cooling of a fatty acid mixture in a crystalliser followed by squeezing the unsaturated fatty acid out of the solidified saturated fatty acid cake with a membrane filter